58 



MISC. PUBLICATION 679, U. S. DEPT. OF AGRICULTURE 



CASTING SUPPORTING BASES FOR SPECIMENS 



In order to support the specimen in the center of the finished block, a 

 preliminary layer of plastic must first be cast in a suitable mold (fig. 13) . 

 The best results in casting methacrylate resins are usually obtained with 

 glass molds because they permit watching the specimen as polymeriza- 

 tion progresses and also easy removal of the block when it has hardened. 

 A mold should be chosen that will yield a cast block of the desired shape 

 and size and is large enough to provide for surrounding the specimen with 

 at least *4 inch of plastic on all sides. 



For making cylindrical castings, Pyrex-type glass beakers are con- 



Figure 13. — A small amount of casting sirup is first poured into the mold and poly- 

 merized to serve as a supporting base for the specimen. 



